1. Field of the Invention
The present invention relates to surface-treated Aluminum or Aluminum alloy materials which excel in adhesive property, formability, weldability, phosphatability, paint adhesion, and post-painting corrosion resistance. These are used in applications wherein they are painted and used after subjected to press forming and other processing, spot or laser welding and phosphate treatment, including panel materials for automobiles and other various vehicles, shells for household electrical apparatus, and building materials.
2. Description of the Prior Art
Aluminum or aluminum alloy materials (hereinafter identified as Al alloy materials) are lightweight and possess superb corrosion resistance and designability, and have found extensive applications as shells for household electrical appliances and building materials.
During recent years, Al alloy materials have come to be employed in automobiles and other vehicles in order to reduce the weight of the body. With this, there have been increasing opportunities for them to be pressed, welded and further painted.
However, since Al alloy materials have their surfaces covered with a stable oxide film (passive state coating), they are poor in adhesion, formability, weldability (spot welding and laser welding) and paintability. They have poorer phosphatability, which is done as a surface treatment before painting, and thus there is the problem that paint adhesion and post-painting corrosion resistance are not improved. In the field of automobiles, the various parts are press-formed into a specified form, assembled with spot welding and laser welding, or joined to the specified locations with adhesive. Al alloy materials possess inferior adhesive property and formability when compared to the ordinary steel plate, and also have inferior spot weldability and laser weldability.
During a phosphating process conducted to improve paintability, Al dissolves from the surface of an Al alloy material into a phosphating bath, and the dissolved Al ion impedes the formation of phosphate coatings on the surface of the metal to be treated. To overcome this problem, in Patent Publication No. 157693/1986, a process is proposed in which Zn or Fe plating is formed on the surface of Al alloy materials, to prevent the dissolution of Al ions. These coated layers have poor adhesive property on Al alloy materials, so that exfoliation of these coatings occurs during press forming or spot welding. Accordingly, Al dissolves, preventing the formation of a fine phosphating coating, or the bare Al surface is oxidized resulting in poor phosphating property, if the waiting time from the press forming to phosphating process is longer.